Superabsorbent polymers are water insoluble, hydrogel-forming polymers capable of absorbing large quantities of aqueous fluids including synthetic urine, brines, and biological fluids such as urine, sweat, and blood, while retaining the absorbed fluids under pressure. Hydrogel-forming superabsorbent polymers are useful as absorbents for water and aqueous body fluids when the polymers are incorporated in absorbent articles, such as disposable diapers, adult incontinence pads, sanitary napkins, and bandages. Many of the existing superabsorbents are formed from unsaturated carboxylic acid monomers including acrylic acid, methacrylic acid, alkylacrylates, and acrylamides which are rendered water insoluble by crosslinking.
The degree of crosslinking affects the absorbent capacity and gel strength of a superabsorbent. Capacity is a measure of the amount of fluid which a given amount of superabsorbent polymer will absorb. Gel strength indicates the tendency of the hydrogel once formed to deform under an applied stress. Polymers exhibiting inadequate gel strength will form a hydrogel which deforms and fills the void space in an absorbent article, inhibiting absorbent capacity and fluid distribution throughout the article. Polymers having low absorbent capacity are incapable of absorbing a sufficient amount of the fluid encountered in use of a diaper or other absorbent article. A polymer having a high gel strength generally possesses a low absorption capacity, and a polymer having a high absorption capacity typically possesses a low absorption rate because of gel blocking phenomenon or low gel strength after absorption.
Another characteristic that a superabsorbent polymer must possess is an acceptable level of extractable, water soluble polymer remaining within the superabsorbent. The extractable polymer can leach out of a hydrogel when fluids contact the superabsorbent. The extractables that leach out of the superabsorbent apparently lower the absorption speed and capacity of the superabsorbent, resulting in leakage of the fluid from an absorbent article.
Commercially available superabsorbents generally possess sufficient capacity, but do not have adequate gel strength, swell rate (i.e., absorption speed) and absorption under pressure for the absorbent articles of reduced size and thickness that are now being produced. As fluff fiber in absorbent products is replaced with greater amounts of superabsorbent polymer, the polymer has to perform the functions of the fluff fiber. The polymer must quickly absorb fluid and transport it throughout an absorbent! article without releasing the stored fluid from the swelled hydrogel on exertion of pressure. Accordingly, the swollen gel particles cannot impede absorption of additional fluid by forming a barrier, but must maintain their liquid permeability.
In order to improve the absorption speed of superabsorbent polymers, blowing agents have been incorporated into superabsorbents as described in U.S. Pat. Nos. 5,118,719 and 5,145,713. As the blowing agent is dispersed throughout the monomer solution during polymerization, it releases carbon dioxide when heated. The porosity of the resultant superabsorbent polymer provides more surface area within the polymer particles, increasing the rate at which fluid is absorbed by the polymer.
The absorption under pressure of a superabsorbent has been improved by crosslinking the molecular chains at the surface of the polymer. Surface crosslinkage also improves the gel strength of the polymer and reduces the amount of extractables at the polymer surface. Although capacity is reduced at the polymer surface, the core of the polymer, which has lower crosslink density, retains its absorbance capacity. Crosslinking at the surface of the polymer particles provides spacings between the particles when swelled, allowing fluid to pass the swelled particles and travel throughout the absorbent article. Surface crosslinkage, however, frequently reduces the absorption speed of the polymer. While these materials possess adequate absorption under pressure, they absorb significantly slower than the fluff fiber they are replacing in thinner personal care articles.
The polymers which have been crosslinked at their surface (herein referred to as core polymers) are not porous materials like those disclosed in U.S. Pat. Nos. 5,118,719 and 5,145,713. Accordingly, these superabsorbents generally exhibit slow rates of absorption. U.S. Pat. Nos. 4,666,983 and 5,140,076 disclose absorbent polymers formed from reacting an absorbent resin powder having a carboxyl group with a crosslinking agent having at least two functional groups per molecule to crosslink the surface of the polymer. German Patent No. 4,020,780 describes surface crosslinked superabsorbent particles formed by coating monomers having acid groups with an alkylene carbonate. U.S. Pat. No. 5,229,466 discloses surface crosslinking by treating a water swellable carboxyl group containing polymer with a solution of an N-(hydroxyalkyl)-beta-(meth)-alanine ester or a polycondensation product thereof. A superabsorbent polymer made from acrylic acid, a water soluble polysaccharide and a crosslinking monomer having at least two polymerizable ethylenically unsaturated double bonds per molecule is described in U.S. Pat. No. 5,145,906. Water absorbent resins surface treated with a polyquaternary amine are disclosed in U.S. Pat. No. 4,824,901.
There is a need for a method of producing a water absorbent resin which exhibits high absorbency under pressure, high absorption speed and high gel strength.